Your pediatrician might be right with not poo-ing for 3-4 days. My friend's baby was on full breastmilk dont't poo for 7 days straight. But that's her child's pattern. Baby do poo less as they grow older.
For watery poo, does it have grains in them? Dark green poo is ok. My baby also has dark green poo.
If your baby's appetite is affected, it could be constipation.